In order to allow the arterial-like free flow of blood through a vein selected for use as an arterial bypass, the surgeon must first disable the vein's valves. The procedure is best accomplished by severing the valves with a valvulotome.
A common problem encountered by vascular surgeons is the restriction of arterial blood flow in the lower extremities. Such restriction usually results from atherosclerotic occlusive disease. If left untreated, tissue necrosis inevitably results, usually requiring amputation of the limb. A favored method of restoring proper arterial blood flow is in situ arterial bypass surgery.
Veins return blood to the heart; venous blood in the lower extremities must overcome the force of gravity to continue its journey to the heart. In addition, venous return flow is not aided by the pressure exerted by contraction of the heart to the extent that arterial flow is so aided. To accomplish the difficult task of returning blood to the heart from the lower extremities, veins have developed specialized strategies.
Both veins and arteries are composed of three layers: an outer connective tissue layer with numerous fibers, a middle layer of smooth muscle, and an inner layer of connective tissue lined with simple squamous endothelium. However, unlike the arterial wall, the venous wall is easily collapsible, as the smooth muscle layer in veins is much less developed, and the connective tissue layer is thicker, than in arteries. Because venous walls easily collapse, contraction of the surrounding skeletal muscles squeezes the veins, thereby creating pressure on the venous blood, just as contraction of the heart creates pressure on arterial blood. This externally generated pressure on venous blood would not result in the blood's return to the heart if veins did not have some mechanism to inhibit retrograde blood flow.
Nature has provided an elegant solution to this problem: one-way valves arranged so that blood will easily flow toward the heart and not reverse course when skeletal muscular contractions squeeze the vein. These valves are actually two semicircular flaps formed from the vein's interior endothelial lining. However, while the valves are necessary for proper venous blood flow, they are no longer desirable when a vein segment is to be used as an arterial bypass to revascularize a limb.
It is, by now, a well-known surgical technique to remove a vein segment from a limb and reversing its direction upon reimplantation so the valves will allow blood to flow away from the heart. However, this solution has created significant problems. First, because blood vessels naturally taper the farther they are from the heart, an inverted bypass segment has its narrower end connected to the wider end of the artery and vice versa. This anomalous structure can create undesirably turbulent blood flow that may be linked to the development of intimal hyperplasia. Second, during the surgical procedure, veins are quite sensitive to handling, lack of blood, and lowered temperature. These trauma can lead to strong contractions resulting in severe venospasms. Finally, the open wounds necessary for this procedure can result in painful, swollen, erythematous, and, often, necrotic wounds.
In response to these drawbacks of the reversed grafting technique, surgeons developed the in situ bypass procedure. This procedure allows the surgeon to revascularize the limb without removing the bypass vein from its natural position. Because the vein will retain its natural orientation, the turbulent blood flow associated with inverted veins is avoided. Also, because the vein is minimally handled and not removed from its natural environment, the incidence of severe venospasms is reduced. Finally, because the leg is not required to be laid open, wound necrosis is minimized. However, there remains the problem of the wrong-way valves.
The preferred solution to this problem is to disable the valves using a valvulotome. Valvulotomes are medical catheters that are inserted into the vein and destroy venous valves, thereby enabling the vein to be used as an arterial bypass. The devices comprise a cable having a head unit sized to fit the diameter of the saphenous vein selected for the bypass. The head unit is usually a blunted cylinder having a rearward-facing cutting blade that shreds the valves when the surgeon withdraws the valvulotome from the vein. The extant devices most similar to this invention are the LeMaitre.RTM. valvulotome and the valvulotome disclosed in U.S. Pat. No. 5,152,771 to Sabbaghian et al. Neither of these devices reads on the presently claimed invention.
My prior invention of circa 1983, the LeMaitre.RTM. retrograde valvulotome (made and sold by Vascutech, Inc.), has become an industry standard. It comprises a tapered head unit with a recessed blade and a tapered follower attached to a cable. To perform the valvulotomy, the surgeon inserts the valvulotome into the saphenous vein selected for the bypass and works the head to a locus just above the first valve to be severed. Upon withdrawal of the valvulotome by the surgeon, the follower opens the collapsed vein to its proper diameter, protecting the vein from the recessed cutting blade. There is enough space between the follower and the cutting blade for the valve to return to its normal closed position, whereafter the cutting blade shears the valve, rendering it inoperative.
However, despite its technical and commercial success, the above-described retrograde valvulotome, as implemented in the present state of the art, does pose several difficulties. First, the surgeon must select an appropriate size valvulotome based upon the vein selected for the bypass. Yet, because the vein naturally expands the farther one goes toward the heart, a valvulotome that is appropriate for the more distal point where the valvulotome is inserted into the vein is often too small for the portion of the vein lying closer to the heart. Thus, for valves significantly "upstream" of the insertion point, the surgeon must turn the valvulotome in order to bias the cutting blade against the portion of the valve closest to the vein wall and ensure that both sides or cusps of the valve are disabled. As can be imagined, success depends greatly upon the surgeon's technique and experience; as often a 2 mm instrument attempts to cut a 6 mm valve. Second, a safety feature is needed whereby the cutting blade can be retracted or sheathed to protect the vein wall if the need arises. Finally, ways are needed for fluid insertion or removal at or near the cutting head and to allow for fluid pressure monitoring at that site by the surgeon during the procedure.
The Sabbaghian valvulotome comprises an orthogonal blade assembly for cutting the venous valves, a shaft for guiding the blade assembly through the bypass vein, an expansion means for preventing the blade assembly from contacting the walls of the bypass vein, and a control means for controlling the expansion and contraction of the expansion means. Although the inventor of this device claims it to be useful in a variety of bypass vein diameter sizes, the surgeon still must control the degree of cutting blade bias against the vein wall. Since veins naturally taper, this places a burden on the surgeon to precisely control the cutting blades, lest the vein wall be damaged or even punctured. However, less than 20% of in situ procedures are performed using an angioscope, which allows the surgeon to visually reference the interior of the vein. Thus, typically, the surgeon has no way of determining the diameter of the vessel at a given point and thus, the precise location of the vein wall. Accordingly, any positional adjustments made by the surgeon are guesses at best.
The ideal valvulotome should effectively, efficiently, and automatically disable the valves, yet not damage the vein wall. Accordingly, it is desirable that the valvulotome be self-centering to minimize the incidence of injuries to the venous wall due to physician error. In addition to enabling precise cutting blade control, the ideal valvulotome should be able to provide optimal results regardless of the size of the vein lumen. This is best achieved by having the cutting blades automatically bias against the vein wall. Further, the ideal valvulotome should have a safety feature whereby the cutting blade can be retracted or sheathed to protect the vein wall if the need arises. Finally, the ideal valvulotome should a/low for fluid insertion or removal at or near the cutting head and also should allow for fluid pressure monitoring at that site by the surgeon during the procedure.
The present invention provides an elegant solution to all of the problems associated with conventional valvulotomes and is the closest approximation yet of an ideal valvulotome.
A principal object of the present invention is to provide a self-centering means for optimally severing venous valve flaps regardless of the vein lumen diameter.
Another object of the present invention is to provide such a valve-severing means that does not pose a significant risk of patient injury due to variations in physician skill and expertise.
Yet another object of the present invention is to provide a valve-severing means whereby the cutting blade can be retracted or sheathed to protect the vein wall if the need arises.
A still further object of the present invention is to provide a valve-severing means that allows for fluid insertion or removal at or near the cutting head and that also allows for fluid pressure monitoring at that site by the surgeon during the procedure.